This invention relates generally to a method and apparatus for regulating the intensity of the catalytic conversion of hydrocarbons and more particularly concerns a method and apparatus for regulating the severity for the conversion of hydrocarbons in the presence of fluidized catalyst particles in a conversion system employing a transfer line reactor.
As a result of the development of catalysts of improved activity, numerous catalyzed hydrocarbon conversion processes can be performed in a transfer line reactor. Such catalysts make possible the utilization of previously recognized advantages of the dilute-phase transport reactor, commonly known as the transfer line or riser reactor. Transfer line reactors have found wide usage in numerous hydrocarbon conversion processes, including in particular the catalytic cracking of hydrocarbons. While the process and the apparatus of the present invention are useful in a wide range of catalytic hydrocarbon conversion processes that can be performed in a transfer line reactor, the present invention will be described hereinafter with particular reference to its application to the catalytic cracking of hydrocarbons.
Transfer line fluid catalytic cracking is a mode of fluid catalytic cracking of hydrocarbons in which finely divided cracking catalyst particles are formed into a suspension with a hydrocarbon feed which substantially is in the vapor phase. In a typical embodiment of transfer line cracking, the suspension flows at a gas velocity in the range between about 15 and 75 feet per second through an elongated reaction zone having a length-to-diameter ratio in the range of from about 10:1 to about 50:1, and the weight ratio of feed per hour to catalyst in the reaction zone is in the range of from about 20:1 to about 300:1. The density of cracking catalyst particles in a typical transfer line or riser fluid catalytic cracking reactor ranges from 1.0 to 10.0 pounds per cubic foot.
Transfer line cracking reactors are increasingly of interest to refiners generally because they feature short residence timers for oil and catalyst which enable active catalyst particles to provide high conversions of cracking feed to hydrocarbons boiling in the gasoline boiling range without excessive overcracking of desirable products. Transfer line reactors are also characterized by good mixing of feed and catalyst and by a lower system catalyst inventory due to more efficient use of the catalyst. Furthermore, the hydrocarbon effluent from a transfer line reactor can be quickly separated from the catalyst so as to avoid excessive conversion of the hydrocarbon conversion product--for example, aftercracking--in a dense bed or even a dilute phase over a dense bed.
In modern refinery practice, it is necessary that the catalytic cracker have maximum flexibility to enable the refiner to adjust product yields and quality and quantity in response to changes in demand. In the development of the fluid catalytic cracking process, it has been recognized that certain feedstocks are more refractory than certain other feedstocks. The more refractory stocks are usually cracked under more severe conditions than the less refractory stocks. In a hydrocarbon conversion process which is performed in a dense fluid-bed reactor system, the reaction severity or the extent of conversion of the hydrocarbon feedstock generally is a function of space velocity which is a measure of the amount of catalyst seen by the oil and the length of time during which the catalyst and oil are in contact. Historically, the space velocity requirement has been accommodated by designing for adequate catalyst inventory in the reactor. Seasonal variations in conversion requirements were met by adjusting the catalyst level in the dense-bed reactor.
Although the need for such flexibility still exists in a system employing a transfer line reactor, the capability of adjusting the catalyst level to enhance the space velocity flexibility as with the dense-bed does not exist with a conventional transfer line reactor. Various apparatus configurations in systems employing a transfer line reactor have been proposed in order to obtain different optimum conversion conditions for feeds of different physical properties. One approach to vary the severity of cracking has been to vary the contact times of feeds having different cracking characteristics with cracking catalysts so that each feed is cracked under process conditions which are optimum for its own cracking characteristics. Typically in conventional systems employing transfer line cracking, more severe cracking conditions can be obtained for the purpose of cracking a more refractory feed by charging the more refractory feed to the transfer line reactor at a point in the transfer line reactor further upstream than the point where a less refractory feed would be charged. Representative systems are disclosed in Woertz, U.S. Pat. No. 2,890,164; Hennig, U.S. Pat. No. 3,065,166; Bryson et al., U.S. Pat. No. 3,617,497; Dober et al., U.S. Pat. No. 3,654,137; Saxton, U.S. Pat. No. 3,671,424; McKinney et al., U.S. Pat. No. 3,692,667; Schwartz et al., U.S. Pat. No. 3,847,793; Strother, U.S. Pat. Nos. 3,948,757 and 4,051,013; and Gross et al., U.S. Pat. No. 4,218,306.
For example, Woertz and Hennig disclose a method in which two feeds, one more and the other less refractory, are charged to a transfer line reactor, with the more refractory feed being charged separately to the transfer line and sufficiently far upstream from the point at which the less refractory feed is charged to the transfer line. This technique permits substantial cracking of the more refractory feed to occur between the points of feed entry. Advantage is thereby taken of the higher temperature and activity of the freshly regenerated catalyst particles further upstream in the transfer line closer to the regenerator where the more refractory feed is introduced than downstream where the less refractory feed is introduced and of the longer contact time of catalyst with the more refractory feed than with the less refractory feed.
An analogous technique involving multiple catalyst injection to a transfer line reactor in order to increase the yield and selectivity of the cracking reaction to gasoline is disclosed in Carr et al., U.S. Pat. No. 3,639,228. Carr et al. disclose a fluid catalytic cracking system employing a transfer line reactor having a short residence time in which only a portion of the fresh or freshly regenerated catalyst which is used is charged to the inlet of the reactor together with the hydrocarbon feed and the remainder of the freshly regenerated catalyst is charged downstream from the transfer line reactor.
Each of these techniques requires (1) injection of either catalyst or oil at a point downstream from the inlet to a transfer line reactor and into an accelerated fluidized mixture moving within the transfer line reactor and (2) injection of a more refractory feed into the transfer line reactor at a point substantially upstream of the point where the less refractory may feed is to be injected. However, such requirements pose major problems to the practical operation of a typical transfer line reactor. First, introduction of either hydrocarbon or catalyst at a point downstream from the inlet to the transfer line reactor into a suspension therein moving at a velocity of 20-60 feet per second can cause a serious pressure imbalance in the system. Secondly, a system requiring introduction of feed at a point substantially upstream of the point where a second feed is to be introduced may necessitate the use of a transfer line reactor which is significantly longer than what is conventionally employed.
Catalytic cracking systems in which combinations of a transfer line reactor and a fluidized bed reactor have been employed to effect different cracking conditions for feeds having different cracking characteristics have also been reported. Typical such combinations are disclosed in Owen, U.S. Pat. No. 3,671,424 and Bunn et al., U.S. Pat. No. 3,784,360. For example, Owen discloses a modification of a fluidized bed catalytic cracker which permits at least a portion of the feed to the cracking reactor to be reacted in a dilute catalyst phase rather than the fluidized bed and at a temperature substantially different from that of the fluidized bed. This is accomplished by dividing the riser conduit into two separate and parallel conduits with one of these conduits having its outlet above the fluidized bed of the reactor and the other having its outlet within the fluidized bed of the reactor. The method thus makes possible the cracking of two different feedstocks simultaneously in a conventional fluidized bed catalytic cracker under different conditions of temperature and catalyst-to-oil ratios. However, any such reported combination in which the effluent from a transfer line reactor is passed into a conventional fluidized bed reactor, eliminates the aforesaid advantage of a transfer line reactor of permitting a quick separation of hydrocarbons from catalyst particles in the effluent and thereby minimizing aftercracking.